1. Field of the Invention
The present invention relates generally to rotating machines such as electric motors and generators and, more particularly, to cooling the field windings of the stator of electric motors, alternators and generators.
2. Prior Art
There have been very many proposals intended to improve the operation of transducers for electrical power/mechanical power conversion (motors, generators, or alternators). However, there are still areas where the use of electric motors remains impractical, for example for use as the main drive of a vehicle such as an automobile. Present electric motors are generally too large, heavy, and produce too little power (especially at high speed) for commercial use in a vehicle such as an automobile.
One problem associated with electrical machines, such as electric motors, is that it is necessary to cool them because they generate heat which reduces their efficiency. Motors that are driven by inverters have a high frequency component of current in their windings due to the high frequency pulse width modulation of the inverter. This high frequency component adds to the losses in the stator winding by both the increased RMS current it represents and by the skin and proximity effects in the wire. Conductor losses in a motor represent a large part of the total losses in a well designed motor.
At present, such machines may be cooled by blowing air through or over them. For heavy duty applications it is known to spray oil onto the rotor and stator assemblies and into the gap between them using a high pressure pump. A scavenger pump may also be provided to collect the sprayed oil for re-cycling. The need for optimization in cooling is even more important as some applications of electric motors demand high efficiency in compact packages.
A common configuration for such motors is to have an inner rotor mounted on a straight shaft supported by bearings on the ends. The bearings are mounted in end covers that support and locate the rotor in the center of a current-carrying stator. The rotor contains multiple current-carrying bars which run length wise parallel to the shaft and are located near the outer circumference of the rotor. Heat is produced in the rotor and stator when the current in the stator excites the bars. Heat dissipation limits the design of the stator.
In a typical electric rotating machine where heat dissipation is required, it is customary to circulate a cooling medium in the outer structural jacket of the machine if the cooling medium is a liquid. Whereas, if the medium is air, the flow is then routed through the center and outside shell of the device.
A transverse cross-section of the stator field windings of a prior-art motor is illustrated in FIG. 1. In this design, the electromagnetic conductors 100 are housed in a slot 102 of a stator 104. The conductors 100 are cooled by the longitudinal flow of a synthetic oil between the interstices of the wires large external radii and the region formed by an interior wall of a slot liner 106. Maintaining this cross-sectional area open along the longitudinal axis of the stator 104 has proven to be very problematic during the manufacturing process of the motor and this creates a high fluid flow pressure drop. This problem ultimately manifests itself when the fixed power output of the oil pump delivery system cannot deliver the designed mass flow of the oil that is needed to sufficiently cool the conductors 100 where enormous heat is generated due to internal electrical resistance.
This particular electric motor designed for an electric vehicular application produces 450 lb-ft of torque and 250 HP at normal steady state conditions but coolant oil leaking from the stator slots into the air gap between the stator and rotor bodies causes hydrodynamic drag which leads to significant power transmission inefficiencies. Furthermore, the incessant impact of the rotor into this entrapped oil causes hardware damage due to cavitation and heat build up. Thus, the cooling oil meant to absorb the heat from the conductors 100 typically leaks from the interstices, causing an obstruction to the mechanical function of the motor.
The process to form these interstices in each slot 102 is tedious and fraught with manufacturing risk. FIGS. 2a to 2d illustrate this. FIG. 2a illustrates Nomex/Kapton/Nomex (NKN) slot liners 106 which are placed in each stator slot 102 followed by a pair of pre-formed solid magnetic conductors 100. A center stick 108 made of a modified fiberglass cloth saturated with a high temperature phenolic resin (PCGP-HT) material placed between them separates the upper and lower conductors 100.
As illustrated in FIG. 2b, four 0.030 inch diameter steel wires 110 are then inserted into the interstitial space between the conductors 100 and slot liners 106. As illustrated in FIG. 2c, a PCGP-HT top stick 112 is then carefully inserted into place. After making the welded connections on the wires and buss ring terminals, the entire stator sub-assembly is then dipped into a bath of Doryl B-109-9 electrical insulating varnish under vacuum. After the excess varnish is drained, the stator sub-assembly is placed in a curing oven.
Referring now to FIG. 2d, half way through the curing process, taking great care so as not to tear the slot liner 106, the four steel wires 110 are then gently pulled out and removed to expose the interstitial oil cooling flow passages 114. The curing process is continued so that the remaining varnish coating will form a secondary dielectric insulation 116 on the magnet wires as well as fuse the slot liner end papers and steel laminates, thus forming a means of primary containment for the cooling oil along with maintaining oil flow passages.
There are major problems associated with this cooling design and methodology. Keeping the interstitial area open along the longitudinal axis of the stator has proven to be very problematic since manufacturing variation in the cross-section tends to create relatively high fluid flow pressure drops. Another issue is that the varnish, being a thin liquid, does not completely fill the gaps between the slot liner end papers, top sticks and the steel laminates, thus compromising the integrity of the oil""s primary containment. However, the biggest issue with this sealing technique is that incurred by the large temperature differential of 140 degrees Centigrade between the inner diameter of the stator and the outside diameter of its aluminum alloy casting. The different thermal coefficients of expansion of the elements, mainly in the stator slots, create a complex system of 3-dimensional expansion and contraction due to this temperature differential of the operating motor. This movement creates such stresses in these sealing joints that the glass-like crystalline structure of the varnish tends to fracture. Thus, these differential thermal expansions and contractions lead to the destruction of any sealing provided by the varnish.
All these problems can be compounded to such a magnitude that the pressurized oil coolant will breach any flaw in its containment and flow radially towards the center of the stator, filling the air gap between the stator and rotor. At the relatively low speeds of 5,000 RPM or less, the cooling medium is churned and ground by the spinning rotor causing a drag force and hence mechanical losses to the output of the motor. At the maximum operating speed of 15,000 RPM, the cooling medium is subjected to heavy churning which not only results in major propulsion inefficiency, but causes high, localized temperatures that breaks down the oil""s viscosity and hence, cooling capacity.
Therefore it is an object of the present invention to provide a rotating machine with cooled stator field windings having internal passages which provide an increased amount of cooling than is provided by prior art methods for cooling rotating machines.
In order to increase heat conduction away from the field windings of a stator of a rotating machine, oil coolant is passed through longitudinal passages defined in at least one of the field windings. The rotating machine of the present invention utilizes a unique design that is much more thermally robust than the traditional solid conductor approach. This principle has been demonstrated to be readily manufacturable and resides within the economic producibility of the motor.
In summary, there is provided an improved rotating machine. The improved rotating machine of the present invention comprises: a stator having a plurality of field winding slots; a plurality of field windings disposed in each of the field winding slots, at least two of the field windings are comprised of: an outer jacket; and a plurality of conductive wires disposed within and enclosed by the jacket such that longitudinal passages are defined therebetween; and circulation means for circulating a coolant into and from the rotating machine through the longitudinal passages.
In a preferred implementation of the rotating machine of the present invention, a housing is further provided. The housing has a cavity for acceptance of the stator therein. The housing and stator define first and second plenums at first and second ends of the stator. The coolant enters the rotating machine into the first plenum and exits the rotating machine from the second plenum.
The at least one field winding having the longitudinal passages preferably has at least one entry hole in the outer jacket which provides communication between the longitudinal passages and the first plenum and at least one exit hole in the outer jacket which provides communication between the longitudinal passages and the second plenum, wherein the coolant enters the longitudinal passages from the first plenum through the at least one entry hole and exits into the second plenum through the at least one exit hole.
The circulation means preferably comprises a pump disposed in an external conduit between the first and second plenums. A heat exchanger is also preferably disposed in the external conduit between the first and second ends for removing heat from the coolant re-circulated therein.
The rotating machine of the present invention further comprises an inpregnant disposed in the slots to seal the spaces between the outer jackets and walls of the slots.